A diesel-engine system may include a soot filter or so-called diesel particulate filter (DPF). The DPF is configured to trap soot entrained in the engine exhaust. The DPF may include a honeycomb arrangement of hollow cells having exhaust-permeable walls. Each cell may extend in length from the inlet end of the filter to the outlet end of the filter. In a typical arrangement, each cell of the DPF is open at one end and closed at the other. Adjacent cells may be oriented alternately, such that a cell having an open inlet and closed outlet is adjacent one or more cells having closed inlets and open outlets. In this configuration, exhaust entering an open inlet of a cell is forced to cross one or more of the exhaust-permeable partitions into a cell having an open outlet.
As a DPF accumulates soot, its capacity for continued accumulation is naturally reduced. Further, a DPF operating with too much accumulated soot will impart excessive backpressure to the exhaust flow, which may degrade engine performance and fuel economy. Therefore, a DPF may be configured to support one or more regeneration modes, which restore the capacity of the filter for continued soot trapping. Some diesel-engine systems are configured to temporarily increase the exhaust temperature to burn away soot accumulated in the DPF. This is done by increasing the amount of energy released into the exhaust as heat. Accordingly, an engine may operate at reduced mechanical efficiency and fuel economy during DPF regeneration. For a given engine system, a DPF of greater capacity requires less frequent regeneration, and so provides improved fuel economy.
However, increasing the capacity of a DPF may require increasing its length, or its diameter, or both. Packing constraints naturally limit the diameter of a DPF in a motor-vehicle engine system, and thermal-management constraints may limit its length. During regeneration, soot accumulated in a DPF burns exothermically. If the accumulated soot is distributed unevenly with respect to the central axis of the filter, then the DPF may heat up unevenly—viz., a large thermal gradient may develop along the axis, resulting in unbalanced mechanical stress. Such stress is known to increase with the ratio of the length of the DPF to its diameter. When the ratio is too high, ring-off crack failure may occur during regeneration.
U.S. Pat. No. 7,204,965 describes a DPF in which a cell structure open on both ends is arranged upstream of the half-open honeycomb structure described above. This configuration addresses the issue of excessive backpressure caused by soot clogging the DPF inlet. The present application, by contrast, describes a DPF system that not only reduces backpressure due to inlet clogging, but also enables improved thermal management during regeneration. More specifically, the system here disclosed is configured so that soot accumulation in the DPF is roughly even in the axial direction at the time of regeneration. In this manner, thermal gradients and mechanical stresses during regeneration are reduced.
Accordingly, one embodiment provides an engine system comprising an envelope configured to transmit exhaust. The envelope encloses a first array of filtration cells downstream of a second array of filtration cells. Each cell of the first array has one open end and one closed end. Each cell of the second array has one or two open ends. The system further comprises a fuel injector configured to increase a temperature in the envelope when soot is evenly distributed between the first and second arrays. Another embodiment provides a method for removing soot from motor-vehicle exhaust using a regenerable soot filter, the filter comprising a first array of filtration cells enclosed in an envelope downstream of a second array of filtration cells. The method comprises trapping the soot in the first array at a first rate and trapping soot in the second array at a second rate, where the second rate decreases faster than the first rate. The method further comprises increasing the temperature in the envelope to oxidize the soot when the soot is evenly distributed between the first and second arrays.
The summary above is provided to introduce in simplified form a selected part of this disclosure, which is further described hereinafter. It is not meant to identify key or essential features of the claimed subject matter. Rather, the claimed subject matter is defined only by the claims and is not limited to implementations that solve any disadvantages noted herein.